The Wireless Local Area Network, WLAN, technology is a general technology for local wireless communications. As the name implies the Wireless Local Area Network, WLAN, technology offers a basis for wireless communications within a local area coverage. The WLAN technology includes industry-specific solutions as well as proprietary protocols, although most commercial applications are based on well-accepted standards such as the various versions of IEEE 802.11, also popularly referred to as Wi-Fi.
WLAN is standardized in the IEEE 802.11 specifications such as IEEE Standard for Information technology—Tele-communications and information exchange between systems. Local and metropolitan area networks—Specific requirements. Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications). WLAN systems following the 802.11 MAC specifications operate based on distributed medium or channel access, meaning that each node in the network has more or less equal probability of accessing the medium.
WLAN or Wi-Fi currently mainly operates on the 2.4 GHz or the 5 GHz band. The IEEE 802.11 specifications regulate the access points' or wireless terminals' physical layer, MAC layer and other aspects to secure compatibility and inter-operability between access points, also referred to as APs, and wireless devices or terminals/stations, also referred to as STAs. Wi-Fi is generally operated in unlicensed bands, and as such, communication over Wi-Fi may be subject to interference sources from any number of both known and unknown devices. Wi-Fi is commonly used as wireless extensions to fixed broadband access, e.g. in domestic environments and hotspots, like airports, train stations and restaurants.
WLAN/Wi-Fi Positioning
WLAN- or Wi-Fi-based positioning system is used where Global Positioning System (GPS) is inadequate due to various causes including multipath and signal blockage indoors. Such systems include indoor positioning systems. WLAN/Wi-Fi positioning takes advantage of the rapid growth in the early 21st century of wireless access points in urban areas.
The localization technique used for positioning with wireless access points is usually based on measuring the intensity of the received signal (received signal strength or RSS) and the method of “fingerprinting”, see further details below. Typical parameters useful to geolocate the WLAN/Wi-Fi hotspot or wireless access point include the Service Set Identifier (SSID) and the Medium Access Control (MAC) address of the access point. The accuracy depends on the number of positions that have been entered into the database. The WLAN/Wi-Fi hotspot database gets filled by correlating mobile device GPS location data with WLAN/Wi-Fi hotspot MAC addresses. The possible signal fluctuations that may occur can increase errors and inaccuracies in the path of the user. To minimize fluctuations in the received signal, there are certain techniques that can be applied to filter the noise.
In the case of low precision, some techniques have been proposed to merge the WLAN/Wi-Fi traces with other data sources such as geographical information and time constraints (i.e., time geography).
The architecture of a WLAN- or Wi-Fi-based positioning system is shown in FIG. 1. This system incorporates one or more access points (APs), an access controller (AC) and a positioning server. The functions of each component are described as follows:                AP(s): One or more APs are utilized to exchange dedicated frames and/or beacons with terminals or stations (STAs) for positioning purpose. The frames/beacons contain positioning-related information, for example, time stamp, path loss information, etc., based on which STAs perform necessary measurements.        AC: The AC delivers configuration information to the AP(s) to control their behaviours for positioning. In the opposite direction, the measurement results collected at the AP(s) are reported to the AC. Then the AC processes the measurement results and reports the processed data to the positioning server. In certain configurations, the APs may also be able to report the collected data directly to the positioning server.        Positioning server: The positioning server calculates the location of the STA based on the reported data and other available information in the database.        
Several techniques have been used for positioning in WLAN/Wi-Fi systems. Those techniques exploit different signal features and may thus require different measurements and apply corresponding algorithms. They can be classified into the following categories:
Received Signal Strength Indicator (RSSI)
In the early version of the IEEE 802.11 standard, the measurement of the distance-dependent signal strength, defined as Received Signal Strength Indication (RSSI), can be used to locate STAs. In principle, the distance between the STA and the AP could be reflected by RSSI based on certain attenuation model. However, RSSI is sensitive to the radio environment and the behaviour of RSSI could be greatly different from the model due to path loss and interference. Hence, RSSI is usually part of the fingerprinting method that searches for a best match between a stored geographical map of radio properties and the measured radio properties. RSSI is an important one among such radio properties.
Time of Arrival (TOA) and Round Trip Time (RTT)
The time measurement based methods measure the travel time between the STA and the AP and translates the travel time into the distance between the pair.
In the standard [1], the Time of Arrival (TOA) method is supported by that the 802.11 specification has standardized the protocol and signalling for time-stamp (difference) measurement. For TOA positioning, there must be at least three such pairs so that the location can be determined at the intersection of the three circles created by the measured distances. In addition, given the TOA difference between STA-AP pairs, other trilateration-based algorithms, for example, hyperbolic trilateration, can also be applied.
Different from TOA, the RTT method can measure the distance without requiring time synchronization between the nodes. It measures the time spent by a specific frame in traveling from a transmitter to a receiver and back again to the transmitter. The main challenge is Non-line-of-sight (NLOS) that brings uncertainty in the time measurement.
Fine Timing Measurement (FTM)
Since IEEE 802.11-2012, the standard has specified the use of the timing measurement frames. A capable STA may transmit timing measurement frames addressed to a peer STA/AP. The higher-layer protocol for synchronizing the local clock time between STAs has been standardized as well.
In the upcoming amendments, the feature of Fine Timing Measurement (FTM) is added [1]. The FTM is characterized by a three-stage procedure including negotiation, FTM implementation and reporting the time-stamp of the previous FTM exchange. The time-stamp resolution is expected to improve to the order of 100 ps from that of 10 ns. This substantially increases the theoretical limitation of the positioning accuracy.
Location Fingerprinting
Instead of determining the distance between the user and the AP, in WLAN or Wi-Fi location fingerprinting, the location of the user is determined by comparing obtained sensing samples to a fingerprint map. The fingerprint map should be constructed in advance in an offline phase and collects necessary fingerprints, i.e. distinct sensing samples/values including RSSI, Angle of Arrival (AOA), TOA, etc., at each reference point in the map. The procedure of map construction usually requires a test STA to perform reference measurements either at each point of a fine coordinate grid, or by “walking around” the AP coverage area to collect sufficient fingerprint data.
However, all the methods described above have their drawbacks and hence there is still a need for an improved method for positioning in WLAN systems.